The intensity of a magnetic field at points varying distances from an electric cable decreases in proportion to the distance. Unlike a general electric apparatus that is used at a position spaced apart from a person by several meters, an electric heating apparatus for keeping a person warm contacts his or her body, that is, is spaced apart from the body by a small distance of about several centimeters. Thus, the magnetic field to which the body is exposed has a density several ten times or several hundred times as high as the general electric apparatus. Therefore, an electric heating cable of such an electric heating apparatus requires a structure that reduces leakage flux. In order to satisfy the requirements, various types of electric heating cables for preventing leakage flux have been developed. Among the electric heating cables, a coaxial electric heating cable for preventing leakage flux which is similar to that of Korean Patent No. 018436 has been widely used. FIG. 1 shows a coaxial electric heating cable, with part of the outer circumferential surface of the cable cut away.
The coaxial electric heating cable includes core wires 11, an insulating inner covering 12, an outer heating wire 13, and an insulating outer covering 14. The core wires 11 are annealed copper stranded wires. The insulating inner covering 12 covers the outside of the core wires 11, and is made of nylon or Teflon, having superior heat resistance. The outer heating wire 13 is spirally wound around the outer circumference of the insulating inner covering 12. The insulating outer covering 14 covers the surface of the outer heating wire 13, and is made of an insulating material, such as silicone or polyvinyl chloride (PVC). The outer heating wire 13 is obtained by forming a resistor, such as a circular or polygonal nichrome wire, in a circular or ribbon shape. Current flows in the core wires 11 and the outer heating wire 13 in opposite directions, thus offsetting magnetic fields, therefore eliminating leakage flux.
Such a coaxial electric heating cable is constructed so that the core wires 11, the insulating inner covering 12, the outer heating wire 13, and the insulating outer covering 14 are layered in multiple layers. Thus, the coaxial electric heating cable is thick and relatively inflexible. Especially, since the insulating inner covering 12 directly applies power to the core wires 11 and the outer heating wire 13, and is present inside the outer heating wire 13, both heat resistance and ability to withstand voltage must be ensured.
Thus, even if fluorine resin, having superior mechanical and thermal properties, is used for the insulating inner covering, a considerable thickness is required. Thereby, the diameter of most practical products exceeds 2.5 mm. Meanwhile, if the insulating outer covering 14 is made of silicone or vinyl chloride, which has relatively lower mechanical strength, the diameter of the product is 3 mm or so. Thus, when the coaxial electric heating cable is applied to a thin electric heating apparatus, the coaxial electric heating cable protrudes from the electric heating apparatus. When the insulating outer covering 14 is made of fluorine resin, thus reducing the diameter of the coaxial electric heating cable, the manufacturing cost is increased, and in addition, productivity is reduced due to high-temperature extrusion.
Further, when external shocks act on the outer heating wire 13, that is, when the coaxial electric heating cable is folded, the outer heating wire is gathered to one side, so that the interval between spiral parts of the outer heating wire is narrowed. Thereby, one surface of the coaxial electric heating cable is overheated, so that the insulating material melts or is damaged. Moreover, the outer heating wire 13 is apt to short, which thus increases leakage flux.
An electric heating cable for suppressing leakage flux in a method different from that of the coaxial electric heating cable has been proposed. FIG. 2 shows the basic construction of a dual insulating twisted electric heating cable, with part of the outer circumferential surface of the cable cut away.
As shown FIG. 2, the dual insulating twisted electric heating cable is manufactured by pulling and rotating two electric heating strands 20, or by pulling and twisting the two electric heating strands 20 while simultaneously rotating the electric heating strands. Each electric heating strand 20 includes core wires 21, which are stranded wires of a resistor, such as a nichrome wire, and an insulating inner covering 22 which is made of high-temperature resin, such as fluorine resin. The twisted electric heating strands 20 are covered with an insulating outer covering 23 so that the electric heating strands are in close contact with each other and do not come untwisted. Current flows in the two electric heating strands 20 in opposite directions, so that magnetic fields are offset with each other, and thus leakage flux is reduced. Korean U.M. Registration No. 0317437 and Korean U.M. Registration No. 0176447 disclose examples implementing the dual insulating twisted electric heating cable.
In such a dual insulating twisted heating cable, the interval between the core wires 21 to which power is applied is equal to twice the thickness of the insulating inner covering 22. Thus, even though the thickness of the insulating inner covering 22 is reduced to the maximum within the range that exhibits desired mechanical and thermal properties, the cable has considerable ability to withstand voltage. Unlike the coaxial electric heating cable, the phenomenon where the outer heating wire 13, which is spirally wound, gathers at one side does not occur. However, the total diameter is still thick, due to dual insulation, and flexibility is poor. Thus, when the dual insulating twisted heating cable is applied to a thin electric heating apparatus, the dual insulating twisted heating cable protrudes. Due to the process of applying the insulating outer covering 23, the dual insulating twisted heating cable is disadvantageous in terms of cost and productivity.
Most of the drawbacks of the coaxial electric heating cable and the dual insulating twisted heating cable are overcome if the twisted heating cable is used without the insulating outer covering 23. However, when a twisted electric heating cable, made by performing rotation with respect to only one axis, is able to freely move without restriction, or the electric heating apparatus in which the twisted electric heating cable is embedded becomes deformed, space is formed between the electric heating strands, and thus leakage flux is not efficiently suppressed.
In order to solve the drawback of the twisted electric heating cable, U.S. Pat. No. 6,734,404 proposes a method for preventing two electric heating strands from coming untwisted via a means for adhering the electric heating strands to each other, thus removing the insulating outer covering 23.
However, the adhering means must adhere a material having a low frictional coefficient, such as fluorine resin, must not be deformed, and must not lose its ability to adhere the electric heating strands, even when temperatures of 100° C. or more are reached in an electric heating apparatus. Further, the adhering means must have tensile strength and flexibility sufficient to withstand mechanical damage when a thin electric heating apparatus is abruptly bent. However, it is very difficult to obtain an adhering means satisfying the above-mentioned requirements. Further, the manufacturing cost is increased because of the adhering means, and productivity is reduced because of the need to apply the adhering means.